drm/amdgpu: mark local function as static

Mark amdgpu_ras_debugfs_create_ctrl_node() as static.

Fixes: eb14235668777b ("drm/amdgpu: do not keep debugfs dentry")
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>